In typical double flow steam turbines, a flow of motive steam is provided through an opening in an outer casing to an inlet chamber in an inner casing, whereupon the steam is directed onto a first pair of annular rows of stationary blades positioned on either side of the middle of the turbine. A number of such rows of stationary blades are fixed to the inner casing through attachment, by any one of several known methods, to a blade ring, of which there are several types. A bearing mounted rotor, having a number of annular rows of blades disposed about the periphery of the rotor, is positioned within the inner casing so that the rotor blades are cooperatively associated with the rows of stationary blades. As the motive steam flows and expands from the turbine middle outward, the stationary blades serve to direct the motive steam past the rotor blades to motivate the rotor in a well known manner.
If a portion of the total flow of steam escapes from or otherwise circumvents the above described flow path, the force associated with that portion of steam will not act on the rotor blades, but rather, will be lost. In such situations, the turbine will not be able to achieve maximum efficiency during operation.
In prior double flow steam turbines, steam circumvents its flow path as a result of several necessary gaps between moving and stationary parts. For example, the first annular rows of stationary blades positioned on either side of the turbine inlet typically have inner shroud rings attached to the end of the blades. Each of these shroud rings includes series of strips, wherein each strip is connected to a number of blades. Since the shroud rings are positioned proximate the rotor, there exists a necessary gap to prevent contact with the rotor. Steam passing through this gap will not motivate the rotor as efficiently as if such steam had been directed by a stationary blade onto an annular row of rotor blades.
In attempting to resolve this problem in the past, so-called "belly bands" were incorporated in turbines having a narrow inlet, i.e. from about 5.50 inches to about 8.00 inches in diameter. In such double flow steam turbines the "belly band" was bolted between the ends of shroud rings positioned on either side of the inlet to prevent any steam from passing between the shroud rings. Unfortunately, while this technique stopped the flow of steam it also introduced unwanted turbulence into the flow of steam, reducing turbine efficiency. Another, and perhaps more significant, problem resulting from the use of "belly bands" was the transfer of incompatible loads generated by inner casing thermal expansion. Such loads would be transferred through the band to the shroud rings and the stationary blades. If the incompatibility of such loads, i.e. the difference in load forces, is significant enough the shroud rings, blades, or blade roots may become deformed.
In double flow steam turbines having a wide inlet, i.e. greater than or equal to about 10.00 inches, a structurally complex devices were incorporated in the inlet to prevent the flow of steam from circumventing the desired flow path. This device included a central band which was suspended in the inlet by various structures attached to either the walls of the inlet chamber or to the blade rings. Other band-like structures were attached to the shroud rings positioned on either side of the inlet, so that each band-like structure frictionally engaged an opposite edge of the central band. Such prior devices were not only very costly but also required significant time for installation.
It should also be kept in mind with regard to the circumvention by steam from the desired path that the thermal expansion and/or contraction which occurs can become significant enough to cause ovalized deformation of the ends of the turbine inner casing. Such ovalized deformation can cause portions of the inner casing to which the stationary blades are attached to move away from the rotor, making any existing gap even larger.
Consequently, there is a need for a double flow steam turbine which includes structure for preventing the flow of steam from circumventing the desired flow path and which structure, at the same time, does not contribute additional turbulence to the flow of steam.